Cryopumps currently available, whether cooled by open or closed cryogenic cycles, generally follow the same design concept. A low temperature second stage array, usually operating in the range of 4 to 25 K, is the primary pumping surface. This surface is surrounded by a high temperature cylinder, usually operated in the temperature range of 70 to 130 K, which provides radiation shielding to the lower temperature array. The radiation shield generally comprises a housing which is closed except at a frontal array positioned between the primary pumping surface and the chamber to be evacuated. This higher temperature, first stage, frontal array serves as a pumping site for higher boiling point gases such as water vapor.
In operation, high boiling point gases such as water vapor are condensed on the frontal array. Lower boiling point gases pass through that array and into the volume within the radiation shield and condense on the second stage array. A surface coated with an adsorbent such as charcoal or a molecular sieve operating at or below the temperature of the second stage array may also be provided in this volume to remove the very low boiling point gases. To prevent overloading of the adsorbent, the adsorbent is generally provided on surfaces which are protected by the second stage condensing array. With the gases thus condensed or adsorbed onto the pumping surfaces, only a vacuum remains in the work chamber.